Communication systems on which data is transmitted are well known in the art. Before a user may transmit a data message, also known as a packet, many systems require a data (communication) unit to first request a communication resource, such as a frequency, frequency pair, TDM (time division multiplexed) time slot, and so forth. The system, by way of a controller, receives the request, also known as a reservation request, allocates a communication resource, and transmits a reservation grant, including information about the allocated communication resource. The allocation provides the user with the exclusive use of the assigned communication resource during an allotted time. Such methods can be very efficient, because the only portion of a data packet subject to collision, i.e., contention with other data units, on the channel is the reservation request, which is generally quite short.
Nevertheless, imperfect channel conditions found in a mobile radio data system can result in degradation of system performance. One such degradation of interest results from the data unit failing to recognize that a reservation grant has been transmitted to it. This failure may be caused by either poor signal-to-noise ratio, co-channel interference, or multipath fading. If the unit fails to recognize the reservation granted to it, the communication resource allocated for that reservation is essentially lost to the system, since all other data units are locked out of that channel for the duration of the reservation. This loss diminishes the access efficiency, thus reducing the overall channel capacity of the system.
While this situation could generally occur in any reservation-based data system, some systems permit a range of data rates to be used in an attempt to optimize throughput by matching the data rate to the current channel conditions. In other words, when a channel has good quality conditions, a higher data rate is used, and when channel conditions are poor, a lower data rate is transmitted. The lowest rate (offering the highest level of error protection) is very inefficient, although it has the highest likelihood of successful receipt. If the lowest data rate used at all times, transmission overall is inefficient in that a minimal amount of information is transmitted in the same time period. While use of a higher data rate for transmission is a more efficient use of channel space, such a data rate has the lowest probability of successful reception over all signal conditions, and consequently is not received with the greatest probability of successful decoding, resulting in retransmission of data messages. To solve the problem of high efficiency of channel versus high probability of receipt, many systems use a middle-of-the-road data rate, i.e., a rate in between the highest and lowest rates available in the system. Even with a medium date rate, data may not be transmitted reliably due to poor channel conditions, and the error rate of the reservation grant can be significant.
Time diversity is a well known method of improving communication reliability. With this method, information is simply repeated a number of times, thus increasing the probability that at least one repetition is received correctly. Such a solution could be applied to the reservation reliability problem by repeating the reservation grant a fixed number of times before actually beginning the reservation period. For use in good signal conditions, however, these repetitions would simply delay their use of the channel thus degrading the quality of service offered.
Accordingly, there is a need for an improved access method that is efficient in both time and communication resource usage.